1. Field of the Invention
This invention relates to a control device, a control method, a control unit, and an engine control unit, for calculating a control input to a controlled object e.g. based on a ΔΣ modulation algorithm so as to control an output from the controlled object.
2. Description of the Prior Art
Conventionally, a control device of the above-mentioned kind has been disclosed e.g. in Japanese Laid-Open Patent Publication (Kokai) No. 2001-154704. The control device includes detection means that detects an output from a controlled object (i.e. controlled system) and then outputs a detection signal of an analog quantity indicative of the sensed output, deviation-calculating means that calculates a deviation between a target value of an analog quantity inputted from a host system and the detection signal, conversion means that converts the calculated deviation to a 1-bit digital signal, and compensation means that performs compensation for the 1-bit digital signal delivered from the conversion means and then inputs the same into the controlled object as an input signal (see FIG. 6 of the publication).
In this control device, the deviation (analog quantity) between the detection signal and the target value is calculated by the deviation-calculating means, the calculated deviation is converted to the 1-bit digital signal through ΔΣ modulation by the conversion means, and then the 1-bit digital signal is compensated for by the compensation means, followed by being inputted into the controlled object as the input signal. In the above process, an operation amount having an opposite phase to the deviation is generated so as to cancel out the deviation between the output from the controlled object and the target value, and inputted into the controlled object. As a result, the output from the controlled object is feedback-controlled to converge to the target value.
According to the above conventional control device, if the dynamic characteristics of the controlled object include relatively large phase delay or dead time, it takes time before an output signal reflecting the input signal is outputted from the controlled object after input of the input signal into the controlled object, which causes lag in control timing between the input and output of the controlled object. As a result, the controllability of the controlled object is degraded, which makes the control system unstable. For instance, when the fuel injection amount of an internal combustion engine is used as an input for controlling the air-fuel ratio of exhaust gases from the engine, it takes time before the air-fuel ratio of the exhaust gases actually changes after execution of fuel injection, which causes degradation of stability and controllability in control of the air-fuel ratio, thereby making unstable the characteristics of exhaust gases purified by a catalyst.
A control technique for compensating for the above-mentioned lag in control timing in a controlled object with significant phase delay or dead time has been disclosed e.g. in Japanese Laid-Open Patent Publication (Kokai) No. 2000-179385. In this control technique, a predicted value of an output from a controlled object, in which response delay due to phase delay or dead time in the controlled object is taken into account, is calculated by an identifier and a state predictor. More specifically, the controlled object having a target air-fuel ratio as an input thereof and an output from an air-fuel ratio sensor as an output thereof is modeled as a discrete-time system model in which the response delay is taken into account. Model parameters of the discrete-time system model are calculated by the identifier, and a predicted value of a deviation between the output from the air-fuel ratio sensor and the predetermined target value is calculated by the state predictor.
However, when the control technique described above is applied to the aforementioned conventional control device, there occurs the following problem: The conventional control device executes control by using a ΔΣ modulation algorithm, and therefore, as a control period (repetition period of control) is shorter, the control become more accurate. In other words, as the control period is set longer, the control accuracy is degraded. However, in the above control technique using the identifier and the state predictor, to set a calculation period of the identifier and the state predictor to be short does not necessarily lead to improvement of accuracy in the calculation of identified values of model parameters and the predicted value, but degrades the calculation accuracy in some cases. For instance, in a situation where changes in the input and output of the controlled object or the target value of the output from the controlled object are small, if the calculation period of the identifier and the state predictor is set short, the values calculated by the identifier and the state predictor cannot appropriately reflect the frequency characteristics of the controlled object, which results in degradation of the calculation accuracy all the worse.